Your first stop should be an encyclopedia or other tailored reference source. Below are links for three general encyclopedias and one reference database, which is a cross-searchable collection of hundreds of specialized encyclopedia entries focused on the ancient world.
After looking through encyclopedias, both general and specialized, you'll need deeper sources. Specialized topical databases, which collate a variety of sources all cross-searchable, are an excellent second step. Here you will find additional specialized encyclopedias, primary sources, academic articles, and more.